Tissue oxygen saturation (SO2) provides a measure of oxygen content in red blood cells. Measurement of SO2 in tissue can be used to assess micro-vascular circulation and oxygen supply to tissue cells arising from certain pathologic conditions such as sepsis and diabetes, for example, which result in impaired vascular blood flow. Tissue SO2 measurements can also be used in exercise physiology, where a mismatch between oxygen demand and supply during periods of exercise can be used to determine an extent of physical conditioning of a subject.
Infrared reflectance measurements can be used for non-invasive, quantitative detection of various chemical species in tissue. For example, interrogation of oxygenated and non-oxygenated hemoglobin in tissue can be made via reflectance measurements of the tissue at wavelengths that fall within a range of about 700-1000 nm. In this wavelength range, many chemical species that may be present in the tissue and which are not of interest interact only weakly with incident radiation, and signals arising from hemoglobin can be isolated from signals that arise from other chemical components. Infrared radiation typically penetrates relatively deeply into tissues, and can be used to probe underneath surface tissues such as skin and fat to measure analytes of interest in deeper muscle and other internal tissues. Suitable systems for performing infrared reflectance measurements in tissue are described, for example, in U.S. Publication Number US 2007/0038041 entitled “SYSTEMS AND METHODS FOR CORRECTING OPTICAL REFLECTANCE MEASUREMENTS,” filed on Apr. 25, 2006, the entire contents of which are incorporated herein by reference.